How SWMM 5 Handles Invert Jumps
An important aspect of how SWMM 5 handles flow routing at nodes where there's a difference in pipe invert elevations between the incoming and outgoing pipes. This is a common scenario in real-world drainage systems.
Understanding Invert Elevations
- Invert: The invert elevation is the lowest point inside a pipe or channel. It essentially defines the "floor" of the conduit.
- Node: In SWMM, a node represents a point where pipes connect, such as a manhole or junction.
How SWMM 5 Handles Invert Jumps
When there's a difference in invert elevations across a node, SWMM 5 uses the following logic:
- Water Surface Elevation: The primary driver of flow in SWMM is the water surface elevation at the node. This elevation is determined by the inflow from upstream pipes and any outflow to downstream pipes.
- Flow Initiation: Even if the invert of an outgoing pipe is higher than the invert of an incoming pipe, flow will not occur in the outgoing pipe until the water surface elevation at the node rises above the outgoing pipe's invert.
- Gravity Flow: Once the water level is high enough, flow will occur from the higher water surface elevation in the node to the lower elevation in the downstream pipe, driven by gravity.
Example
Imagine a manhole where an incoming pipe has an invert elevation of 10 meters, and the outgoing pipe has an invert elevation of 10.5 meters.
- Initially, if the water level in the manhole is below 10.5 meters, there will be no flow in the outgoing pipe, even though water is entering from the incoming pipe.
- As inflow continues, the water level in the manhole will rise.
- Once the water level exceeds 10.5 meters, flow will begin in the outgoing pipe.
Key Considerations
- Backwater Effects: This difference in invert elevations can create backwater effects, where the higher invert of the downstream pipe can cause water to back up in the upstream pipe.
- Surcharging: If the inflow exceeds the outflow capacity, the node may surcharge, meaning the water level rises above the crown of the pipes. SWMM 5 can simulate surcharge conditions.
- Energy Losses: There are typically energy losses associated with these invert changes, which SWMM 5 accounts for in its calculations.
Practical Implications
Understanding how SWMM 5 handles invert jumps is crucial for:
- Accurate Modeling: It ensures that the model realistically represents the flow behavior in the drainage system.
- Design Considerations: Engineers can use this knowledge to design systems that avoid excessive backwater effects or surcharging.
- Troubleshooting: If unexpected flow patterns are observed in the model results, checking for invert differences at nodes can be a helpful troubleshooting step.
By correctly representing invert elevations and understanding how SWMM 5 handles these situations, you can create more reliable and accurate models for stormwater management.
